1920-21.] Size, a Neglected Factor in Stelar Morphology. 25 
stem-structure in the hypocotyl, the diameter of the axis is a most 
important factor. He found that low transitions are characteristic of 
massive hypocotyls, high transitions of those which are slender. Here 
again it was not possible to put forward exact measurements, but the 
principle emerges from the average taken from a large number of seedlings 
of different species. The correlation in that case was, however, between 
size and the readjustment of intra-stelar tissues : the form of the stele itself 
was throughout approximately cylindrical. Questions of permeability of 
the endodermal barrier will hardly have affected them. That size may 
thus be correlated on the one hand with the internal disposition of the 
stelar tissues, and on the other with the actual conformation of the stele 
itself, shows how important it may be in determining features which are 
habitually used in comparison. The effect should be to impose caution in 
drawing phylogenetic or classificatory conclusions from characters so 
pliable : for at once the door is opened for frequent homoplasy in stocks 
phyletically quite distinct from one another. Thus the recognition of the 
principle of similar structures and its consequences, while it involves 
interesting points in physiology and anatomy, may react finally even upon 
classification. 
(Issued separately December 30 , 1920 .) 
